1. Field of the Invention
The present invention relates to an image pickup device, and more particularly to an image pickup device provided with electronic zooming means and electronic image vibration compensating means, adapted for use in a video camera or the like.
2. Related Background Art
In an image pickup device for converting an optical image into an electrical signal, the image vibration or the object movement can be detected by a calculation between the frames of the image signal, and the image vibration can be reduced by means of the detected movement vector. The prevention of image vibration can be achieved by an optical configuration in which the vertical angle of a variable-angle prism is controlled according to the detected image vibration, thereby optically compensating the image vibration, or an electronic configuration in which a partial image compensating the image vibration is extracted from an image stored in an image memory and is released as an output in the normal size. The latter is also known as the electronic zooming function for electronically enlarging the taken image.
FIG. 1 is a block diagram of a video camera provided with a circuit for electronically preventing or compensating the image vibration. An image pickup device 12 converts an optical image, formed by a zoom lens 10, into an electrical signal, and an A/D converter 14 converts the output of said image pickup device 12 into a digital signal. An image processing circuit 16 is functionally composed of a memory control circuit 20 for controlling an image memory 18, a movement amount detection circuit 22 for detecting the image movement from the image signal, and an electronic zooming circuit 24, and serves to reduce the image vibration by means of these circuits.
In the above-explained configuration, the image information entered through the zoom lens 10 and the image pickup device 12 is sampled with a predetermined sampling frequency, then converted into a digital signal by the A/D converter 14 and supplied to the image memory 18 through the image processing circuit 16. Then amounts x, y of movement in the horizontal and vertical directions are determined by the correlation of said signal with the next field or frame, and a control circuit 4 reads the image information from the image memory 18, with a reference point displaced according to said amounts of movement.
Referring to FIG. 2, an area W indicates the storage area of the image signal in the image memory 18 and is always constant. An area R.sub.0 is the read-out area of the image signal from the image memory 18 when said amounts of movement are zero, while an area R.sub.xy is the read-out area of the image signal when said amounts of movement are X and Y respectively in the horizontal and vertical directions. The image signal thus read from the image memory is compensated for the vibration and provides a stabler image which is more comfortable to watch.
Within the entire image W stored in the image memory 18, the image processing circuit 16, when the image vibration compensating function is activated, reads a central portion R.sub.0 from the image memory 18 and expands said portion to the size W by the electronic zooming circuit 24, for supply to an interpolation circuit 26.
When the movement amount detection circuit 22 detects a movement of the image, the area of extraction from the entire area W is moved in a direction compensating said movement, for example to an area R.sub.xy shown in FIG. 2, and the extracted image is expanded to the size of the area W by the electronic zooming circuit 24 and is subjected to interpolation between the lines or between the pixels in the interpolation circuit 26.
Thus the interpolation circuit 26 replenishes the pixel data which have become deficient by the expansion in the electronic zooming circuit 24, and a D/A converter 28 converts thus interpolated image data into an analog signal. Thus the output of the D/A converter 28 is a moving image signal with reduced image vibration. A video encoder 30 converts the output of the D/A converter 28 into a predetermined format, for example an NTSC video signal. The output of the D/A converter 28 is also supplied to an electronic view finder (EVF) 32, for image display. The EVF 32 may also be given the output of the video encoder 30.
The above-explained configuration is however unable to completely compensate the image vibration when it is large, so that the target object may vibrate significantly within the image frame or may disappear from the image frame and re-appear therein.
Also in the above-explained configuration, since the image signal already subjected to compensation of image vibration is displayed on the monitor, the photographer tends to misunderstand that the vibration caused by his hands is limited and depends on the image vibration compensating function even though he is in a situation capable of suppressing the vibration of the hands. For example in case of image recording with a camcorder, there is recorded an image with vibration which should have been avoided.
Also in the image vibration compensating device explained above, the read-out image is displaced in parallel manner corresponding to the vibration of the image, and the image has to be expanded in order to avoid the lack of image at an end portion, resulting from such image displacement. Stated differently, the image is expanded when the compensation for image vibration is in action.
If said compensation for image vibration is turned on and off, there will result deterioration of the image quality by a sudden change in the image angle, caused by the presence and absence of said image expansion.